Make sure Flutter is compiled against stable version

.travis.yml

@@ -14,7 +14,7 @@ matrix:
             - libstdc++6
             - fonts-droid
       before_script:
-        - git clone https://github.com/flutter/flutter.git $HOME/flutter
+        - git clone --single-branch --branch stable https://github.com/flutter/flutter.git $HOME/flutter
         - export PATH=$HOME/flutter/bin:$HOME/flutter/bin/cache/dart-sdk/bin:$PATH
         - flutter doctor
       script:

scripts/before_build_apks.sh

@@ -20,7 +20,7 @@ unzip -qq gradle-4.1-bin.zip -d $HOME/gradle-4.1
 export GRADLE_HOME=$HOME/gradle-4.1
 export PATH=$GRADLE_HOME/bin:$PATH
 gradle -v
-git clone https://github.com/flutter/flutter.git $HOME/flutter
+git clone --single-branch --branch stable https://github.com/flutter/flutter.git $HOME/flutter
 export PATH=$HOME/flutter/bin:$HOME/flutter/bin/cache/dart-sdk/bin:$PATH
 flutter doctor
 

scripts/before_build_ipas.sh

@@ -5,6 +5,6 @@ brew install ideviceinstaller
 brew install ios-deploy
 pod repo update
 gem update cocoapods
-git clone https://github.com/flutter/flutter.git $HOME/flutter
+git clone --single-branch --branch stable https://github.com/flutter/flutter.git $HOME/flutter
 export PATH=$HOME/flutter/bin:$HOME/flutter/bin/cache/dart-sdk/bin:$PATH
 flutter doctor