fix conditions for project gemset

When gemset name has a '.'(dot) after the 2nd character, it is project gemset.
Insted it might want to be common gemset.

For example: 
$ echo 'example.1' > .rbenv-gemsets
$ gem env
...
  - GEM PATHS:
     - /path/tp/project/example.1
     - /usr/local/rbenv/versions/2.1.2/gemsets/global
...

I hope for:
$ gem env
...
  - GEM PATHS:
     - /usr/local/rbenv/versions/2.1.2/gemsets/example.1
     - /usr/local/rbenv/versions/2.1.2/gemsets/global
...