RUBY_REVISION breaks rvm

[meatballhat]

per discussion in #514 the subprocess call to get the RUBY_REVISION string doesn't play nice with rvm. os.chdir to the topaz working copy before making the call should Fix It.

[chrblabla]

Seems like RVM doesn't keep the .git directory after executing rvm install topaz --head:

vagrant@precise32:~/.rvm/src/topaz-head$ ls -la
total 92
drwxr-xr-x 11 vagrant vagrant 4096 Mar 19 19:27 .
drwxrwxr-x 25 vagrant vagrant 4096 Mar 19 19:27 ..
-rw-rw-r--  1 vagrant vagrant  362 Mar 19 19:26 AUTHORS.rst
drwxrwxr-x  2 vagrant vagrant 4096 Mar 19 19:26 bench
drwxrwxr-x  2 vagrant vagrant 4096 Mar 19 19:26 bin
-rw-rw-r--  1 vagrant vagrant  415 Mar 19 19:26 CONTRIBUTING.rst
drwxrwxr-x  4 vagrant vagrant 4096 Mar 19 19:26 docs
-rw-rw-r--  1 vagrant vagrant   17 Mar 19 19:26 .gitignore
drwxrwxr-x  2 vagrant vagrant 4096 Mar 19 19:26 lib-ruby
drwxrwxr-x  2 vagrant vagrant 4096 Mar 19 19:26 lib-topaz
-rw-rw-r--  1 vagrant vagrant 1536 Mar 19 19:26 LICENSE
-rw-rw-r--  1 vagrant vagrant  818 Mar 19 19:26 README.rst
-rw-rw-r--  1 vagrant vagrant   54 Mar 19 19:26 requirements.txt
drwxrwxr-x  3 vagrant vagrant 4096 Mar 19 19:26 spec
-rw-rw-r--  1 vagrant vagrant  332 Mar 19 19:26 targettopaz.py
-rw-rw-r--  1 vagrant vagrant  916 Mar 19 19:27 targettopaz.pyc
drwxrwxr-x  2 vagrant vagrant 4096 Mar 19 19:26 tasks
drwxrwxr-x  6 vagrant vagrant 4096 Mar 19 19:26 tests
drwxrwxr-x  6 vagrant vagrant 4096 Mar 19 19:27 topaz
-rw-rw-r--  1 vagrant vagrant 9975 Mar 19 19:26 topaz.mspec
-rw-rw-r--  1 vagrant vagrant  705 Mar 19 19:26 .travis.yml