Fix CoffeeScript linter path processing

The path was not being created properly for coffee_lint. This commit
emulates the other plug-ins by creating a paths variable.

Submitted as a pull request here:

  https://github.com/causes/overcommit/pull/18

Signed-off-by: Greg Hurrell <greg@causes.com>
Change-Id: I3a1a64621bd22942cdc0402114776153107fa408
Reviewed-on: https://gerrit.causes.com/27244
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>
Replace native 'which' with Ruby version

Addresses issue #15:

  https://github.com/causes/overcommit/issues/15

Cross-platform path search from:

  http://stackoverflow.com/questions/2108727/which-in-ruby-checking-if-program-exists-in-path-from-ruby

Originally submitted as a pull request here:

  https://github.com/causes/overcommit/pull/17

with a couple of minor amendments applied:

  - fixed broken method name (was `#in_path`, but all the call-sites use
    `#in_path?`)
  - use `do`/`end` rather than curlies for multi-line blocks
  - remove unnecessary explicit `return`
  - return `true`/`false` rather than `String`/`nil`, as none of the
    call-sites use the returned string

Signed-off-by: Greg Hurrell <greg@causes.com>
Change-Id: I8cc5ecea9eca1b0c2a2c3b482c4b5ccb2aa7dfcc
Reviewed-on: https://gerrit.causes.com/27248
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>