Add python-yaml to Travis CI environment
Add python-yaml to Travis CI environment

Signed-off-by: Pekka Enberg <add4fcd06328a394f0ad91feda7ee057316dc5ed@iki.fi>

mstanichenko/libtrading,penberg/libtrading,penberg/libtrading,svdev/libtrading,jvirtanen/libtrading,etoestja/libtrading,Bitcoinsulting/libtrading,divaykin/libtrading,mstanichenko/libtrading,libtrading/libtrading,NunoEdgarGub1/libtrading,fengzhyuan/libtrading,etoestja/libtrading,divaykin/libtrading,NunoEdgarGub1/libtrading,fengzhyuan/libtrading,Bitcoinsulting/libtrading,femtotrader/libtrading,jvirtanen/libtrading,svdev/libtrading,libtrading/libtrading,femtotrader/libtrading