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
Fix scratch-buffer alignment warnings
[coretext] More scratch buffer fixes
More scratch-buffer cleanup
Use long alignment for scratch buffer

Fixes last of scratch alignment warnings in hb-coretext.
0.9.24
Set buffer content type to INVALID in hb_buffer_set_length(0)

Previously we were only setting this in hb_buffer_clear_contents(),
but set_length(0) is a valid way to reinitialize buffer to use with
new text.
Merge branch 'master' of git://github.com/behdad/harfbuzz
JOHNE - Bump the version number to 0.9.24