cache ta-lib folder, skip build if cache exists

.travis.yml

@@ -33,3 +33,4 @@ notifications:
 cache:
   directories:
     - $HOME/.cache/pip
+    - ta-lib

install_ta-lib.sh

@@ -1,3 +1,8 @@
+if [ ! -d "ta-lib" ]; then
   wget http://prdownloads.sourceforge.net/ta-lib/ta-lib-0.4.0-src.tar.gz
   tar zxvf ta-lib-0.4.0-src.tar.gz
   cd ta-lib && ./configure && make && sudo make install && cd ..
+else
+  echo "TA-lib already installed, skipping download and build."
+  cd ta-lib && sudo make install && cd ..
+fi