A few Travis CI fixes

- Switch to using ghdl/vunit:llvm, it's a smaller container
- We need to "apt update" before installing packages

Signed-off-by: Anton Blanchard <anton@linux.ibm.com>

.travis.yml

@@ -3,6 +3,6 @@ install: skip
 
 services: docker
 
-before_install: docker pull ghdl/vunit:gcc
+before_install: docker pull ghdl/vunit:llvm
 
-script: docker run --rm -t -v `pwd`:/build -w /build ghdl/vunit:gcc bash -c "apt install -y python3-pexpect && make GNATMAKE='gnatmake -j'$(nproc) && make -j$(nproc) check_light"
+script: docker run --rm -t -v `pwd`:/build -w /build ghdl/vunit:llvm bash -c "apt update && apt install -y python3-pexpect && make GNATMAKE='gnatmake -j'$(nproc) && make -j$(nproc) check_light"