travis: Create build matrix for each target

* Build for each target
* Remove the `-j` flag due to each build VM only having "1.5" cores

.travis.yml

@@ -1,5 +1,17 @@
+env:
+  - TARGET=CC3D
+  - TARGET=CHEBUZZF3
+  - TARGET=CJMCU
+  - TARGET=EUSTM32F103RC
+  - TARGET=MASSIVEF3
+  - TARGET=NAZE
+  - TARGET=NAZE32PRO
+  - TARGET=OLIMEXINO
+  - TARGET=PORT103R
+  - TARGET=SPARKY
+  - TARGET=STM32F3DISCOVERY
 language: c
 compiler: gcc
 before_install: sudo add-apt-repository -y ppa:terry.guo/gcc-arm-embedded && sudo apt-get update
 install: sudo apt-get install build-essential gcc-arm-none-eabi git
-script: make -j4
+script: make