[CONAN] Update dependencies

[Croydon]

• Switch Boost back to lasote's package (we don't want to maintain it permanently)
• Update gtest to 1.8.0 and exclude gmock, to fix GCC 5.4 under some circumstances
  • also excluding stuff we don't need = always good
• GlueGen switched to stable, SDL & CEF switched to testing
  • why => https://github.com/inexorgame/inexor-core/issues/415#issuecomment-309252210

Since this causes that we have packages for all recipes for all platforms/compilers/configs we do support except SDL and CEF, the build time goes down dramatically, e.g. in Travis it only takes 20 minutes now vs 30 minutes. In AppVeyor it should be even more saved time, but I didn't test it.

@a-teammate this CEF version is now the latest from our repository, do you have still problems with this version?

[Croydon]

Windows linking problems right now. Don't merge yet.

[a-teammate]

whats the state of this? Why is travis failing, do you got clue already?

[Croydon]

GCC is fixed by upgrading. Also you can see now the promised time drop from 30mins to 20 :)

Still the windows linking problems.