Fix Github Action and modify supported xcode/macOS versions in both Github Action and Travis CI. (#563)

* Modify xcode version for both Travis CI and Github Action

* Drop support for Golang 1.13 and add 1.15

* Drop support for Golang 1.13 and add 1.15 for Travis as well
diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 80b72c3..3736183 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -16,9 +16,9 @@
         working-directory: ${{ env.WORKING_DIR }}
     strategy:
       matrix:
-        go: ['1.13', '1.14']
+        go: ['1.14', '1.15']
         os: ['macos-10.15']
-        xcode-version: ['11.6', '11.5', '11.4.1', '11.3.1', '11.2.1', '11.1', '11.0', '10.3']
+        xcode-version: ['12.0', '11.7', '11.6', '11.5', '11.4.1', '11.3.1', '11.2.1', '11.1', '11.0']
     steps:
       - name: Update go version
         uses: actions/setup-go@v2
@@ -70,7 +70,7 @@
         working-directory: ${{ env.WORKING_DIR }} 
     strategy:
       matrix:
-        go: ['1.13', '1.14']
+        go: ['1.14', '1.15']
         os: ['ubuntu-20.04', 'ubuntu-18.04', 'ubuntu-16.04'] 
     steps:
       - name: Update go version
@@ -114,7 +114,7 @@
     runs-on: windows-2019
     strategy:
       matrix:
-        go: ['1.13', '1.14']
+        go: ['1.14', '1.15']
     steps:
       - name: Update go version
         uses: actions/setup-go@v2
diff --git a/.travis.yml b/.travis.yml
index e34cbcf..42e16c1 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -12,26 +12,26 @@
       osx_image: xcode10.3
       go: master
     - os: osx
-      osx_image: xcode11
-      go: master
+      osx_image: xcode10.3
+      go: 1.14.x
     - os: osx
-      osx_image: xcode11.1
-      go: master
-    - os: osx
-      osx_image: xcode11.2
-      go: master
+      osx_image: xcode10.3
+      go: 1.15.x
     - os: osx
       osx_image: xcode11.3
       go: master
     - os: osx
-      osx_image: xcode11.4
-      go: master
+      osx_image: xcode11.3
+      go: 1.14.x
     - os: osx
-      osx_image: xcode11.5
-      go: master
+      osx_image: xcode11.3
+      go: 1.15.x
     - os: osx
       osx_image: xcode11.6
       go: master
+    - os: osx
+      osx_image: xcode12
+      go: master
 
 addons:
   apt: