Update charlock_holmes to v0.7.7

charlock_holmes v0.7.7 resolves a C++ build issue (https://github.com/brianmario/charlock_holmes/compare/0.7.6..v0.7.7) on some machines.

This may also resolve a CentOS 6 build issue (omnibus-gitlab#5682 (closed)) since the gem will be available for gitaly-ruby using the newer compiler.