.travis.yml: avoid Valgrind bug with '-march=native'

The switch in #163 to `-march=native` exposed a Valgrind bug on the Travis CI machines, see e.g. [build 436](https://travis-ci.org/miloyip/rapidjson/builds/38787102) and following:
````
vex amd64->IR: unhandled instruction bytes: 0xC4 0xE1 0xFB 0x2A 0x45 0x80 0xEB 0x1A
==3841== valgrind: Unrecognised instruction at address 0x418d6b.
````
Possibly fixed in Valgrind 3.10, see upstream bug https://bugs.kde.org/show_bug.cgi?id=326469.

This pull-request temporarily changes the `-march=native` flags back to `-msse4.2` after the generation of the gmake files.  I still think that the original change in #163 is reasonable.