Fix up Travis CI and Python version support

[rayrrr]

• Tie Travis CI to jazzband instance
• Remove EOL 3.3 and 3.4 version support
• Add 3.7 version support

The broken URL "details" link for the Travis CI build on #119 led me to discover these issues.

[codecov]

Codecov Report

Merging #120 into master will increase coverage by 0.54%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #120      +/-   ##
==========================================
+ Coverage   92.58%   93.13%   +0.54%     
==========================================
  Files          11       11              
  Lines         364      364              
  Branches       70       70              
==========================================
+ Hits          337      339       +2     
+ Misses         17       16       -1     
+ Partials       10        9       -1

Impacted Files	Coverage Δ
geojson/utils.py	83.8% <0%> (+1.9%)	⬆️

---

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[codecov]

Codecov Report

Merging #120 into master will increase coverage by 1.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #120      +/-   ##
==========================================
+ Coverage   92.03%   93.13%   +1.09%     
==========================================
  Files          11       11              
  Lines         364      364              
  Branches       70       70              
==========================================
+ Hits          335      339       +4     
+ Misses         18       16       -2     
+ Partials       11        9       -2

Impacted Files	Coverage Δ
geojson/_version.py	100% <100%> (ø)	⬆️
geojson/utils.py	83.8% <0%> (+3.8%)	⬆️

---

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[rayrrr]

At this point, there is just some PyPy Travis CI weirdness to clear up. I might have done too many pushes in too short of time so going to revisit tomorrow.

[galuszkak]

@rayrrr I fixed this PR for you. Hope You will find it useful.

Thanks!