Fixing compiler package for clang

[etr]

hotfix.

Travis has moved to xenial so all the packages referring to specific trusty toolchains are failing. This change aims to fix it by moving to specific xenial toolchains.

[codecov-io]

Codecov Report

Merging #152 into master will decrease coverage by 1.24%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #152      +/-   ##
=========================================
- Coverage   96.55%   95.3%   -1.25%     
=========================================
  Files          34      34              
  Lines        3019    3153     +134     
=========================================
+ Hits         2915    3005      +90     
- Misses        104     148      +44

Impacted Files	Coverage Δ
src/httpserver/string_response.hpp	40% <0%> (-60%)	⬇️
src/httpserver/http_response.hpp	66.66% <0%> (-33.34%)	⬇️
test/integ/ws_start_stop.cpp	88.34% <0%> (-11.66%)	⬇️
src/details/http_endpoint.cpp	100% <0%> (ø)	⬆️
src/digest_auth_fail_response.cpp	100% <0%> (ø)	⬆️
src/basic_auth_fail_response.cpp	100% <0%> (ø)	⬆️
src/httpserver/create_webserver.hpp	100% <0%> (ø)	⬆️
src/http_request.cpp	96.69% <0%> (+0.26%)	⬆️
src/httpserver/http_request.hpp	95.83% <0%> (+1.38%)	⬆️
src/webserver.cpp	90.28% <0%> (+1.43%)	⬆️
... and 3 more

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