ci: build zig in release mode

It makes sense to test release mode, plus we're up against the
time limits of CI, so this should make room.

ci/travis_linux_script

@@ -8,7 +8,7 @@ export CXX=clang++-6.0
 echo $PATH
 mkdir build
 cd build
-cmake .. -DCMAKE_INSTALL_PREFIX=$(pwd)
+cmake .. -DCMAKE_BUILD_TYPE=Release
 make -j2 install
 ./zig build --build-file ../build.zig test
 

ci/travis_osx_script

@@ -5,7 +5,7 @@ set -e
 
 mkdir build
 cd build
-cmake .. -DCMAKE_PREFIX_PATH=/usr/local/opt/llvm@6/ -DCMAKE_INSTALL_PREFIX=$(pwd)
+cmake .. -DCMAKE_PREFIX_PATH=/usr/local/opt/llvm@6/ -DCMAKE_BUILD_TYPE=Release
 make VERBOSE=1
 make install