New pyomo version breaks oemof tests
If you upgrade to the newest pyomo version (> 5.x) the oemof-tests break. This is not caused by an error but an API change. The equations in the lp-files are still equivalent (see example). 

```diff
 c_e_Bus_balance(electricityBus_1)_:
-+1 flow(electricityBus_excess_1)
-= 720000
+-1 flow(electricityBus_excess_1)
+= -720000
```

1. We could change the rtd_requirements.txt to pyomo >= 5.0 and change the lp-file. This will be fast, does not harm anybody and will fix the Travis-CI problems. In that case you have to make sure that you use an environment with pyomo > 5.0 if you run the tests locally. 

2. We could also set the setup.py requirements to pyomo >= 5.0. This solves everything but is not true. People using pyomo in any other case will have to upgrade pyomo to use oemof even though oemof works fine with pyomo 4.x. In the long run we should do that if we do the tests with pyomo >= 5.0. People might want to upgrade to 5.x anyway.

@oemof/oemof-solph Do you agree to do (1.) now and (2.) later?

diff --git a/examples/oemof_full_check.py b/examples/oemof_full_check.py
index 44aeffb1..15a9d42b 100644
--- a/examples/oemof_full_check.py
+++ b/examples/oemof_full_check.py
@@ -185,7 +185,12 @@ def run_example_checks():
         'date_from': '2030-01-01 00:00:00',
         'date_to': '2030-01-14 23:00:00',
         'nodes_flows': 'example_energy_system.csv',
-        'nodes_flows_sequences': 'example_energy_system_seq.csv', }
+        'nodes_flows_sequences': 'example_energy_system_seq.csv',
+        'results_path': os.path.join(os.path.expanduser("~"), 'csv_dispatch'),
+    }
+
+    if not os.path.isdir(testdict[key]['results_path']):
+        os.mkdir(testdict[key]['results_path'])
 
     try:
         res = dispatch.run_example(config=testdict[key])
diff --git a/rtd_requirements.txt b/rtd_requirements.txt
index 42981a64..47d5919b 100644
--- a/rtd_requirements.txt
+++ b/rtd_requirements.txt
@@ -1,5 +1,5 @@
 dill
 numpy >= 1.7.0
 pandas >= 0.18.0, <0.19
-pyomo >= 4.2.0, != 4.3.11377
+pyomo >= 5.0.0
 matplotlib
diff --git a/tests/lp_files/fixed_source_invest_sink.lp b/tests/lp_files/fixed_source_invest_sink.lp
index e50d8f29..4a92eecc 100644
--- a/tests/lp_files/fixed_source_invest_sink.lp
+++ b/tests/lp_files/fixed_source_invest_sink.lp
@@ -11,16 +11,16 @@ objective:
 s.t.
 
 c_e_Bus_balance(electricityBus_0)_:
-+1 flow(electricityBus_excess_0)
-= 12000000
+-1 flow(electricityBus_excess_0)
+= -12000000
 
 c_e_Bus_balance(electricityBus_1)_:
-+1 flow(electricityBus_excess_1)
-= 16000000
+-1 flow(electricityBus_excess_1)
+= -16000000
 
 c_e_Bus_balance(electricityBus_2)_:
-+1 flow(electricityBus_excess_2)
-= 14000000
+-1 flow(electricityBus_excess_2)
+= -14000000
 
 c_u_InvestmentFlow_max(electricityBus_excess_0)_:
 -0.80000000000000004 InvestmentFlow_invest(electricityBus_excess)
diff --git a/tests/lp_files/fixed_source_variable_sink.lp b/tests/lp_files/fixed_source_variable_sink.lp
index 79d1cf33..c398a4aa 100644
--- a/tests/lp_files/fixed_source_variable_sink.lp
+++ b/tests/lp_files/fixed_source_variable_sink.lp
@@ -10,16 +10,16 @@ objective:
 s.t.
 
 c_e_Bus_balance(electricityBus_0)_:
-+1 flow(electricityBus_excess_0)
-= 430000
+-1 flow(electricityBus_excess_0)
+= -430000
 
 c_e_Bus_balance(electricityBus_1)_:
-+1 flow(electricityBus_excess_1)
-= 720000
+-1 flow(electricityBus_excess_1)
+= -720000
 
 c_e_Bus_balance(electricityBus_2)_:
-+1 flow(electricityBus_excess_2)
-= 290000
+-1 flow(electricityBus_excess_2)
+= -290000
 
 c_e_ONE_VAR_CONSTANT: 
 ONE_VAR_CONSTANT = 1.0
['examples/oemof_full_check.py', 'tests/lp_files/fixed_source_invest_sink.lp', 'tests/lp_files/fixed_source_variable_sink.lp', 'rtd_requirements.txt']