Expand and correct osx versions.

xcode7.2 doesn't seem to exist anymore and there doesn't
seem to be other 10.10 images available.

Author: jrmuizel

.travis.yml

@@ -3,14 +3,21 @@ language: rust
 rust: stable
 matrix:
   include:
-    # macOS 10.12
-    - env: TARGET=x86_64-apple-darwin CGFEATURES="--features elcapitan"
-    # macOS 10.10
-    - osx_image: xcode7.2
+    # macOS 10.11
+    - osx_image: xcode7.3
       env: TARGET=x86_64-apple-darwin CGFEATURES=""
+    # macOS 10.12
+    - osx_image: xcode9.2
+      env: TARGET=x86_64-apple-darwin CGFEATURES="--features elcapitan"
     # macOS 10.13
     - osx_image: xcode9.4
       env: TARGET=x86_64-apple-darwin CGFEATURES="--features highsierra,elcapitan"
+    # macOS 10.14
+    - osx_image: xcode11.3
+      env: TARGET=x86_64-apple-darwin CGFEATURES="--features highsierra,elcapitan"
+    # macOS 10.15
+    - osx_image: xcode12.2
+      env: TARGET=x86_64-apple-darwin CGFEATURES="--features highsierra,elcapitan"
 
 install:
   - rustup target add $TARGET