(maint) Disable warning register-deprecated.

This warning is specific to Clang, and marks register as deprecated.
Boost still uses register, and we're unlikely to (plus it doesn't
cause any harm when it's used), so the warning is now disabled.
Merge pull request #87 from MikaelSmith/maint/master/Disable-register-deprecated-warning

(maint) Disable warning register-deprecated.