[Travis] Python 3.6 osx builds are broken

[guedou]

See https://travis-ci.org/secdev/scapy/jobs/348646654

This PR attempts to fix Python3.* installation with homebrew.

[codecov-io]

Codecov Report

Merging #1207 into master will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1207      +/-   ##
==========================================
+ Coverage   84.73%   84.79%   +0.06%     
==========================================
  Files         158      159       +1     
  Lines       38244    38245       +1     
==========================================
+ Hits        32406    32431      +25     
+ Misses       5838     5814      -24

Impacted Files	Coverage Δ
scapy/tools/__init__.py	100% <0%> (ø)
scapy/asn1/ber.py	82.28% <0%> (+0.28%)	⬆️
scapy/layers/tls/basefields.py	80.27% <0%> (+0.68%)	⬆️
scapy/layers/tls/handshake_sslv2.py	92.8% <0%> (+0.75%)	⬆️
scapy/layers/tls/record.py	92.08% <0%> (+0.87%)	⬆️
scapy/layers/tls/record_sslv2.py	88.82% <0%> (+1.67%)	⬆️
scapy/layers/tls/keyexchange.py	84.76% <0%> (+1.71%)	⬆️
scapy/main.py	72.2% <0%> (+1.91%)	⬆️