Add ruby updates from @nicolasleger

.travis.yml

@@ -1,6 +1,7 @@
 language: ruby
 sudo: false
 dist: trusty
+before_install: gem update --system
 addons:
   postgresql: "9.6"
 matrix:
@@ -15,7 +16,7 @@ matrix:
       - database/init-user-db-travis.sh
     after_success:
       - coveralls
-  - rvm: 2.4.1
+  - rvm: 2.5.0
     env:
       - LABEL=unit_tests
     services:
@@ -23,7 +24,7 @@ matrix:
     before_script:
       - chmod u+x database/init-user-db-travis.sh
       - database/init-user-db-travis.sh
-  - rvm: 2.3.4
+  - rvm: 2.3.6
     env:
       - LABEL=unit_tests
     services:
@@ -31,7 +32,7 @@ matrix:
     before_script:
       - chmod u+x database/init-user-db-travis.sh
       - database/init-user-db-travis.sh
-  - rvm: 2.2.7
+  - rvm: 2.2.9
     env:
       - LABEL=unit_tests
     services:
@@ -39,7 +40,7 @@ matrix:
     before_script:
       - chmod u+x database/init-user-db-travis.sh
       - database/init-user-db-travis.sh
-  - rvm: 2.4.1
+  - rvm: 2.5.0
     env:
       - LABEL=docker_build_tests
     services: