Rbenv default ruby issue on install
I am trying to install MDL v0.11.0

i get the following error during install 
```
pre-commit
[INFO] Installing environment for git://github.com/markdownlint/markdownlint.
[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...
An unexpected error has occurred: CalledProcessError: command: ('/usr/local/bin/bash', '/Users/abuxton/.rbenv/shims/gem', 'build', 'mdl.gemspec')
return code: 1
expected return code: 0
stdout: (none)
stderr:
    rbenv: version `default' is not installed (set by RBENV_VERSION environment variable)
```

seems to be the same issue as https://github.com/pre-commit/pre-commit/issues/369 on a previous version of pre-commit
pre-commit.log https://gist.github.com/abuxton/d29e6265f3230f838a746d7a5defd5e3 

i have tried, ensuring local rbenv, I have tried pre-commit clean/uninstall and reinstall to no success, mdl will `brew install mdl` successfully and works fine. 

diff --git a/pre_commit/languages/ruby.py b/pre_commit/languages/ruby.py
index 1a0f0c7..81bc954 100644
--- a/pre_commit/languages/ruby.py
+++ b/pre_commit/languages/ruby.py
@@ -52,7 +52,6 @@ def get_env_patch(
     else:  # pragma: win32 no cover
         patches += (
             ('RBENV_ROOT', venv),
-            ('RBENV_VERSION', language_version),
             (
                 'PATH', (
                     os.path.join(venv, 'gems', 'bin'), os.pathsep,
@@ -61,6 +60,9 @@ def get_env_patch(
                 ),
             ),
         )
+    if language_version not in {'system', 'default'}:  # pragma: win32 no cover
+        patches += (('RBENV_VERSION', language_version),)
+
     return patches
 
 
['pre_commit/languages/ruby.py']