Simplify tox config, drop python 3.3 & django 1.8

[rpkilby]

A couple of simplifications in the tox config here:

• there isn't a need to specify a 'restframework' factor, as we only test against the latest version
• no need for a verbose latest, since we're testing the latest versions of each package
• dropped python 3.3 * django 1.8, as DRF 3.7 no longer supports them.

[codecov-io]

Codecov Report

Merging #787 into develop will decrease coverage by 0.49%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           develop     #787     +/-   ##
==========================================
- Coverage    98.36%   97.86%   -0.5%     
==========================================
  Files           15       15             
  Lines         1220     1220             
==========================================
- Hits          1200     1194      -6     
- Misses          20       26      +6

Impacted Files	Coverage Δ
django_filters/compat.py	71.42% <0%> (-14.29%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 978e9ef...f9da6ff. Read the comment docs.