Switch to GCC 7 on Travis, disable currently non-functional ARM + Windows builds

Summary:
Use of deleted function declaration broke older C++11 compilers.
(https://github.com/Bitcoin-ABC/bitcoin-abc/issues/98)
This enables GCC 7.2.0 from Ubuntu's toolchain repository.

Travis unfortunately offers limited support for newer versions
of the toolchains we need - but with this changeset we can at least
get Linux x86_64, i686 and Apple cross-compilation builds working again.

ARM and Windows builds are non-functional and have been removed for now.
To get them to work we would need improved versions of at least:

• g++-arm-linux-gnueabihf
• g++-mingw-w64-i686
• g++-mingw-w64-x86-64

It is better to have Travis working for a subset of platforms than
to have it constantly fail the entire suite, even if short/medium term
it seems we need to move away from Travis due to its lack of currency.

Test Plan: check Travis build results

Reviewers: #bitcoin_abc, deadalnix, schancel

Reviewed By: #bitcoin_abc, schancel

Differential Revision: https://reviews.bitcoinabc.org/D626