Travis Quick Fix + Minor Submodule Updates

## Info (Edited)
This commit fixes several Travis-Related Warnings/Errors/Deprecations in MacOS(Now running on Sierra instead of El Capitan). These Issues included Path Not Set Warnings, shell_session warnings, and a few others.

This Commit also Updates the following Submodules:
- enet (Includes Minor CMakeList Changes for cleaner Solutions)
- Nihstro (Includes a bugfix on compiling with Boost v1.66.0)

.travis.yml

@@ -24,7 +24,7 @@ matrix:
     - os: osx
       env: NAME="macos build"
       sudo: false
-      osx_image: xcode7.3
+      osx_image: xcode9.2
       install: "./.travis/macos/deps.sh"
       script: "./.travis/macos/build.sh"
       after_success: "./.travis/macos/upload.sh"

externals/enet

@@ -1 +1 @@
-Subproject commit a84c120eff13d2fa3eadb41ef7afe0f7819f4d6c
+Subproject commit 39a72ab1990014eb399cee9d538fd529df99c6a0

externals/nihstro

@@ -1 +1 @@
-Subproject commit 7e24743af21a7c2e3cef21ef174ae4269d0cfdac
+Subproject commit 28ac07a5273b6250cfafb4630bee8da820ad33d7