Use gcc-5 to compile everything.

Zimsplit use ofstream move `operator=()` or `swap()` method.
This has been implemented in gcc-5 only
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54316)

So install g++-5 and use it in travis CI.

.travis.yml

@@ -2,6 +2,8 @@ language: cpp
 dist: trusty
 sudo: required
 before_install:
+- eval "${MATRIX_EVAL}"
+- ${CXX} --version
 - openssl aes-256-cbc -K $encrypted_eba2f7543984_key -iv $encrypted_eba2f7543984_iv
   -in travis/travisci_builder_id_key.enc -out travis/travisci_builder_id_key -d
 - chmod 600 travis/travisci_builder_id_key
@@ -23,6 +25,9 @@ deploy:
   on:
     condition: ( "$TRAVIS_EVENT_TYPE" = "cron" )
 env:
+  global:
+    - MATRIX_EVAL="CC=gcc-5 && CXX=g++-5"
+  matrix:
     - PLATFORM="native_dyn"
     - PLATFORM="native_static"
     - PLATFORM="win32_dyn"
@@ -43,3 +48,8 @@ notifications:
     on_failure: always
 addons:
   ssh_known_hosts: download.kiwix.org
+  apt:
+    sources:
+      - ubuntu-toolchain-r-test
+    packages:
+      - g++-5