[travis] Add multiarch support

.travis/build

@@ -18,7 +18,9 @@ mkdir -p "$install_dir"
 
 # Setup environment variables.
 export LD_LIBRARY_PATH="$install_dir/lib:$LD_LIBRARY_PATH"
+export LD_LIBRARY_PATH="$install_dir/lib/`dpkg-architecture -qDEB_BUILD_MULTIARCH`:$LD_LIBRARY_PATH"
 export PKG_CONFIG_PATH="$install_dir/lib/pkgconfig:$PKG_CONFIG_PATH"
+export PKG_CONFIG_PATH="$install_dir/lib/`dpkg-architecture -qDEB_BUILD_MULTIARCH`/pkgconfig:$PKG_CONFIG_PATH"
 
 # Retrieve jrl-mathtools
 echo "--> Compiling jrl-mathtools"