Fix llvm warnings on Mac

Patch from Scott Fleischman.  Warnings were:

harfbuzz/src/hb-font-private.hh:121:42: Implicit conversion loses
integer precision: 'long long' to 'hb_position_t' (aka 'int')
harfbuzz/src/hb-font-private.hh:126:42: Implicit conversion loses
integer precision: 'long long' to 'hb_position_t' (aka 'int')
harfbuzz/src/hb-font-private.hh:400:85: Implicit conversion loses
integer precision: 'long long' to 'hb_position_t' (aka 'int')
harfbuzz/src/hb-ot-layout-common-private.hh:1115:37: Implicit conversion
loses integer precision: 'long long' to 'int'
harfbuzz/src/hb-ft.cc:421:97: Implicit conversion loses integer
precision: 'unsigned long long' to 'int'
harfbuzz/src/hb-ft.cc:422:97: Implicit conversion loses integer
precision: 'unsigned long long' to 'int'
[travis] Enable graphite2