Travis: use correct option for configure

When using an installed version of wxWidgets one has to use --with-wx-prefix instead of --with-wxdir which is expecting the source directory.

.travis.yml

@@ -81,4 +81,4 @@ script:
 - if [[ "${BOINC_TYPE}" == "client" ]]; then ( ./configure --disable-server --disable-manager && make ) fi
 - if [[ "${BOINC_TYPE}" == "apps" ]]; then ( ./configure --enable-apps --disable-server --disable-client --disable-manager && make ) fi
 - if [[ "${BOINC_TYPE}" == "libs-mingw" ]]; then ( cd lib && CC=/usr/bin/x86_64-w64-mingw32-gcc; CXX=/usr/bin/x86_64-w64-mingw32-g++; make -f Makefile.mingw ) fi
-- if [[ "${BOINC_TYPE}" == "manager" ]]; then ( ./3rdParty/buildLinuxDependencies.sh && ./configure --disable-server --disable-client --with-wxdir=./3rdParty/buildCache/linux && make ) fi
+- if [[ "${BOINC_TYPE}" == "manager" ]]; then ( ./3rdParty/buildLinuxDependencies.sh && ./configure --disable-server --disable-client --with-wx-prefix=./3rdParty/buildCache/linux && make ) fi