Update to a more recent version of rocket and to nightly-2018-07-17

[lthms]

This is a prerequisite for Plume-org/Plume#205

I had to update the code in order to deal with a breaking change in rocket. Tests still pass, but I would advice a careful code review nonetheless :).

With this change, I have been able to make Plume build with nightly-2018-07-17.

[codecov-io]

Codecov Report

Merging #1 into plume will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##            plume       #1      +/-   ##
==========================================
+ Coverage   96.77%   96.78%   +0.01%     
==========================================
  Files           6        6              
  Lines        1022     1026       +4     
==========================================
+ Hits          989      993       +4     
  Misses         33       33

Impacted Files	Coverage Δ
src/csrf_fairing.rs	96.13% <100%> (+0.04%)	⬆️

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[lthms]

Should be good with latest changes. Tests run just fine on my laptop, at least.

[lthms]

Thanks!