Update travis to use more recent GCC

[fmassa]

This should enable using newer GCC, which is required for the C++ extensions to work.

This has been factored out from #826 , without installing pytorch-nightly, because now PyTorch 1.1 has been released.

cc @soumith

[codecov-io]

Codecov Report

Merging #891 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #891   +/-   ##
=======================================
  Coverage   56.61%   56.61%           
=======================================
  Files          38       38           
  Lines        3432     3432           
  Branches      540      540           
=======================================
  Hits         1943     1943           
- Misses       1371     1372    +1     
+ Partials      118      117    -1

Impacted Files	Coverage Δ
torchvision/datasets/imagenet.py	21.55% <0%> (ø)	⬆️

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