Remove old Ruby version from CI config & cross build rake task

- also, be sure to cross compile for 2.6

Author: aharpervc

.travis.yml

@@ -8,7 +8,6 @@ env:
     - TESTOPTS="-v"
     - TINYTDS_UNIT_HOST=localhost
 rvm:
-  - 2.3.8
   - 2.4.5
   - 2.5.3
   - 2.6.1

appveyor.yml

@@ -43,8 +43,6 @@ environment:
   TINYTDS_UNIT_AZURE_PASS:
     secure: fYKSKV4v+36OFQp2nZdX4DfUpgmy5cm0wuR73cgdmEk=
   matrix:
-    - ruby_version: "23-x64"
-    - ruby_version: "23"
     - ruby_version: "24-x64"
     - ruby_version: "24"
     - ruby_version: "25-x64"

tasks/native_gem.rake

@@ -8,7 +8,7 @@ task 'gem:windows' => ['ports:cross'] do
   build = ['bundle']
 
   # and finally build the native gem
-  build << 'rake cross native gem RUBY_CC_VERSION=2.5.0:2.4.0:2.3.0:2.2.2:2.1.6:2.0.0 CFLAGS="-Wall" MAKE="make -j`nproc`"'
+  build << 'rake cross native gem RUBY_CC_VERSION=2.6.0:2.5.0:2.4.0 CFLAGS="-Wall" MAKE="make -j`nproc`"'
 
   RakeCompilerDock.sh build.join(' && ')
 end