Clear rbenv variables before starting tmux

Relates to #99
Update to RSpec 3.0.
Changed which... > /dev/null to which -s ...

OS X 10.10 seems to have done something causing which tmux > /dev/null to return false

Which does however have an option for not outputting what it found but return 0 or 1 depending on if it was found, achieving the same goal.
Changed which -s to type

Linux doesn't seem to have -s with the which binary.
[Closes #234, Closes #235] Only call package_name thor versions >= 0.18.x
[Closes #232, Closes #233] Clean up gemspec.

- avoid dependency on git
- variable rename to avoid warning
Merge pull request #236 from thorerik/patch-1

Change from using which to type.
[Closes #239] Corrected direction of version comparison when setting package_name
[Closes #185, Closes #144] Optional project root. Defaults to an empty string.
[#185, #144] Fix optional project root in templates.
Merge pull request #208 from juggler/patch-1

Clear rbenv variables before starting tmux
Update readme with correct Fish completions path.
Merge pull request #247 from ElijahLynn/fish-completion-d

Update readme with correct Fish completions path.
Escape path string in order to handle paths with spaces, parentheses etc.
Merge pull request #251 from hgrimelid/master

Escape path string in order to handle paths with spaces, parentheses etc...
Fix: project is overwrited regardless user choice

FileUtils.copy_file will overwrite the file.
Now, if user choose other than "yes", open the exist project config.
Don't need
Merge pull request #254 from alal/patch-1

Fix project is overwrited regardless of user's choice
Escape project root path once and only once

Fixes #256
Merge pull request #257 from TSFoster/project-root-escaping

Escape project root path once and only once