Update osx_image to xcode12 (#162)

* Install osx_image: xcode12
* Solve config warnings from Travis CI

Author: dnerini

.travis.yml

@@ -6,7 +6,7 @@ env:
     - OPTIONAL_DEPENDENCIES="dask pyfftw cartopy h5py PyWavelets pygrib"
     - TEST_DEPENDENCIES="pytest pytest-cov codecov"
 
-matrix:
+jobs:
   include:
     - os: linux
       python: 3.8
@@ -16,7 +16,7 @@ matrix:
 
     - os: linux
       python: 3.6
-      env: 
+      env:
         - PYTHON_VERSION=3.6
         - RUN_TESTS=true
 
@@ -34,34 +34,32 @@ matrix:
 
     - os: osx
       language: generic
-      osx_image: xcode10.1
+      osx_image: xcode12
       env:
         - PYTHON_VERSION=3.8
         - RUN_TESTS=false
 
     - os: osx
       language: generic
-      osx_image: xcode10.1
-      env: 
+      osx_image: xcode12
+      env:
         - PYTHON_VERSION=3.6
         - RUN_TESTS=true
 
     - os: osx
       language: generic
-      osx_image: xcode10.1
+      osx_image: xcode12
       env:
         - PYTHON_VERSION=3.7
         - RUN_TESTS=true
 
     - os: osx
       language: generic
-      osx_image: xcode10.1
+      osx_image: xcode12
       env:
         - PYTHON_VERSION=3.8
         - RUN_TESTS=true
 
-sudo: false
-
 before_install:
   # Install gcc for MacOSX
   # ----------------------
@@ -129,4 +127,3 @@ script:
 
 after_success:
   - if [[ "$RUN_TESTS" == true ]]; then codecov; fi
-