Update Ruby to 2.5

[maciej-szlosarczyk]

Continuation of #546. Should be done before December 2018 when 2.6 is scheduled to be released.

To upgrade to 2.5, we need:

• Upgrade Rails to 4.2.10 Upgrade Rails to 4.2.10 #826
• Upgrade Devise (currently latest 4.4.3 worked just fine)
• Remove que_mailer - tracked with Replace que_mailer #895
• Fix Disallow invoice with zero amount / Balance top-up with zero should fail #651, which uses BigDecimal in the wrong way.
• Upgrade Grape, as it throws multiple deprecation warning on Fixnum/Bignum -> Integer issue. I know we have Consider another API backend #691, but getting on a Ruby version with security update is far more critical.
• Upgrade Rails to 5.0 Upgrade to Rails 5.0 #377
• Upgrade Rails to 5.1
• Upgrade Ruby itself
• Check mina (mina-0.3.7/lib/mina/exec_helpers.rb:102: warning: constant ::Fixnum is deprecated)

[teadur]

As of Rails 4.2 is not tested against Ruby 2.5 by upstream ( https://travis-ci.org/rails/rails/builds/516718585 ) we should not consider it supported by Rails 4.2.

Same goes for Rails 5.0 ( https://travis-ci.org/rails/rails/builds/516718340 )
First rails version what is tested against ruby 2.5 is 5.1 ( https://travis-ci.org/rails/rails/builds/516044024 )

If possible (we have enough time before Ruby 2.4 end of security support 2020-03)
We should try to upgrade to Rails 5.1 before upgrading Ruby.

If we get time constrained we should upgrade Ruby before getting to Rails 5.1
So for now blocked by #377

[teadur]

PG gem needs to be upgraded with rails 5.1 #896

[vohmar]

rails raised to 5.1.7
ruby to 2.6.3

closed by #1495