[docs/build] adding make -j4 as the default for make, including in the Travis/Appveyor builds, should make build times faster

.travis.yml

@@ -32,7 +32,7 @@ install:
     - if [ "$CC" = "gcc" ]; then export CC="gcc-4.8"; fi
 script:
     - ./configure --datadir=$(pwd)/data
-    - make
+    - make -j4
     - if [[ $DICTIONARIES_CHANGED -ne 0 ]]; then ./src/build_address_dictionary; fi;
     - if [[ $NUMEX_CHANGED -ne 0 ]]; then ./src/build_numex_table; fi;
     - if [[ $TRANSLIT_CHANGED -ne 0 ]]; then ./src/build_trans_table; fi;