Add spring support

This patch is different from hirocaster's in
(https://github.com/hirocaster/rspec-mode/commit/1a06e50f1f6c940e6d998253c0ba81add5aad738)

- Symmetric with other options (checking file existence)
- Use spring if possible
- Do not prefix both spring and zeus even if both are working
Merge remote-tracking branch 'tomyk/support_spring'

Conflicts:
	rspec-mode.el
Do not use 'bundle exec' with Zeus and Spring

This is not recommended. Both tools call Bundler themselves.
Do not use Rake with Spring

Because 'spring rake spec' doesn't work.

Also rename `rspec-use-rake-flag' to `rspec-use-rake-when-possible'.
Fix #49
Bump the version, update change log