Don't run travis on master or 10.15

10.15 is now covered by Github actions and master is unneeded
because of auto

.travis.yml

@@ -1,6 +1,7 @@
 os: osx
 language: rust
 rust: stable
+if: branch != master OR type != push
 matrix:
   include:
     # macOS 10.11
@@ -15,9 +16,6 @@ matrix:
     # 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