.travis.yml: Prune the majority of remaining -dev packages (#3923)
.travis.yml: Prune the majority of remaining -dev packages (#3923)

Remove flex and bison too, since anything needing those
should explicitly depend on them.

Closes Linuxbrew/homebrew-core#3922

Signed-off-by: Bob W. Hogg <c772a964fd55352a3510e5d535dd9ccc9ac30168@linux.com>
Linuxbrew/homebrew-core,maxim-belkin/homebrew-core,rwhogg/homebrew-core,maxim-belkin/homebrew-core,LinuxbrewTestBot/homebrew-core,rwhogg/homebrew-core,Linuxbrew/homebrew-core,LinuxbrewTestBot/homebrew-core