Minor corrections for is_i?

- Rename is_i to more idiomatic is_i?
- Remove unnecessary !! in lieu of idiomatic truthy/falsey
- Simplify regex to eliminate +/-
- Change anchors ^ => \A, $ => \z (^/$ should almost never be used)
Merge pull request #22 from m1foley/minor_is_i_corrections

Minor corrections for is_i?