Run apt update before installing

.travis.yml

@@ -56,6 +56,7 @@ install:
     fi
   - if [ $ARCH = x86_64 ]; then
     if [ $TRAVIS_OS_NAME = linux ]; then
+    sudo apt update;
     sudo apt install musl-tools;
     elif [ $TRAVIS_OS_NAME = osx ]; then
     brew tap altkatz/homebrew-gcc_cross_compilers;