Install raqm on Travis

[radarhere]

Includes raqm in the non-docker Travis jobs. Suggested in #3835 (comment)

[hugovk]

Great, thank you!

We don't have #3942 merged yet, but the skipped numbers are ~50 lower, and coverage is up a bit:

PyPy3 Xenial master
============ 1251 passed, 102 skipped, 2 warnings in 120.12 seconds ============
https://travis-ci.org/python-pillow/Pillow/jobs/553724645#L4556

->

PyPy3 Xenial PR
============ 1302 passed, 51 skipped, 2 warnings in 113.38 seconds =============
https://travis-ci.org/python-pillow/Pillow/jobs/554150718#L4830

---

3.7 Xenial master
============= 1267 passed, 86 skipped, 2 warnings in 35.13 seconds =============
https://travis-ci.org/python-pillow/Pillow/jobs/553724646#L4535

->

3.7 Xenial PR
============= 1318 passed, 35 skipped, 2 warnings in 35.63 seconds =============
https://travis-ci.org/python-pillow/Pillow/jobs/554150719#L4806