Add travis_retry to commands fetching deps.

[kzemek]

https://blog.travis-ci.com/2013-05-20-network-timeouts-build-retries

[codecov]

Codecov Report

Merging #1836 into master will increase coverage by 1.62%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1836      +/-   ##
==========================================
+ Coverage   72.39%   74.02%   +1.62%     
==========================================
  Files         297      298       +1     
  Lines       27738    28649     +911     
==========================================
+ Hits        20082    21208    +1126     
+ Misses       7656     7441     -215

Impacted Files	Coverage Δ
src/jingle_sip/jingle_sip_helper.erl	50% <0%> (-36.67%)	⬇️
src/jingle_sip/sip_to_jingle.erl	55.66% <0%> (-31.94%)	⬇️
src/jingle_sip/mod_jingle_sip_backend.erl	43.26% <0%> (-31.74%)	⬇️
src/jingle_sip/jingle_sip_callbacks.erl	59.31% <0%> (-30.32%)	⬇️
src/jingle_sip/mod_jingle_sip.erl	53.16% <0%> (-27.93%)	⬇️
src/jingle_sip/jingle_to_sdp.erl	48.57% <0%> (-27.84%)	⬇️
..._distrib/mod_global_distrib_outgoing_conns_sup.erl	73.91% <0%> (-8.7%)	⬇️
src/mod_bosh_socket.erl	80.66% <0%> (-1%)	⬇️
src/mongoose_iq.erl	28.57% <0%> (ø)
src/ejabberd_sm.erl	83.38% <0%> (+0.33%)	⬆️
... and 31 more

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[arcusfelis]

ok