.travis.yml: Increase the timeout to 45 minutes

The clang sanitizer tests, and there especially the unicode tests, can
hit the default timeout of 25 minutes (1500 seconds) quite easily, so
let's raise the timeout to 45 minutes (2700 seconds).

.travis.yml

@@ -333,7 +333,7 @@ script:
   # compile and execute unit tests
   - mkdir -p build && cd build
   - cmake .. ${CMAKE_OPTIONS} -DJSON_MultipleHeaders=${MULTIPLE_HEADERS} -GNinja && cmake --build . --config Release
-  - ctest -C Release -V -j
+  - ctest -C Release --timeout 2700 -V -j
   - cd ..
 
   # check if homebrew works (only checks develop branch)