common.gypi: bump to OSX backwards compat to 10.6

When working on this branch under both llvm and gcc the binary was
failing in very odd ways. GDB was showing multiple returns from
functions, line numbers weren't matching, etc. However, the makefile
build worked fine.

Investigating further we found that bumping MACOSX_DEPLOYMENT_TARGET to
10.6 fixes the problem.

common.gypi

@@ -153,7 +153,7 @@
           'GCC_THREADSAFE_STATICS': 'NO',           # -fno-threadsafe-statics
           'GCC_VERSION': '4.2',
           'GCC_WARN_ABOUT_MISSING_NEWLINE': 'YES',  # -Wnewline-eof
-          'MACOSX_DEPLOYMENT_TARGET': '10.4',       # -mmacosx-version-min=10.4
+          'MACOSX_DEPLOYMENT_TARGET': '10.6',       # -mmacosx-version-min=10.6
           'PREBINDING': 'NO',                       # No -Wl,-prebind
           'USE_HEADERMAP': 'NO',
           'OTHER_CFLAGS': [