Test building without 3.0 compatibility on Travis CI too

This will test errors like the one fixed by the previous commit by
testing that samples and tests still build without 3.0 API.

.travis.yml

@@ -24,7 +24,7 @@ jobs:
           name: wxGTK 2 UTF-8 Ubuntu 18.04
         - dist: bionic
           compiler: gcc
-          env: wxGTK_VERSION=3 wxCONFIGURE_FLAGS="--enable-cxx11 --enable-stl" wxMAKEFILE_CXXFLAGS=-std=c++11 wxMAKEFILE_LDFLAGS=-Wl,--no-as-needed wxUSE_XVFB=1
+          env: wxGTK_VERSION=3 wxCONFIGURE_FLAGS="--enable-cxx11 --disable-compat30 --enable-stl" wxMAKEFILE_CXXFLAGS=-std=c++11 wxMAKEFILE_LDFLAGS=-Wl,--no-as-needed wxUSE_XVFB=1
           name: wxGTK 3 STL Ubuntu 18.04
         - dist: bionic
           compiler: clang