Add ignore file support to P4Ruby.
This change adds
three new methods:
P4#ignore_file - Return current ignore file
P4#ignore_file= - Set ignore file name
P4#ignored? - Test if a path is ignored
It also removes the check which objected to the use of Ruby 2.0
since we now support it. Now we simply check that you're not
using 2.1 (still a development release).