Fix linters

Summary:
Two fixes:
1. if cpplint is not present on the system, don't return a confusing error in the linter
2. Add include_alpha, which means our includes should be sorted lexicographically

Test Plan: Tried unsorting our includes, lint complained

Reviewers: rven, ljin, yhchiang, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28845