Update travis.yml

[rajat2004]

Update to Ubuntu 18.04
Install rsync in docker conatiner
Remove extra update, fix comments

[rajat2004]

I'm not sure why docker is being used inside Travis, don't think that's required, especially since the containers are not being pushed or anything

AirSim/.travis.yml

Lines 14 to 18 in 0c8dc72

	- os: linux
	dist: trusty
	sudo: required
	services:
	- docker

Host distribution can be updated to xenial or bionic, not sure if it really matters, so haven't added it

Currently, Travis builds aren't triggering, could the maintainers please try to have a look at it? Thanks!

I've started looking into enabling Windows build on Travis, branch is here - master...rajat2004:travis-windows
MSBuild error seems to be fixed, but it's complaining about Visual Studio, need to look into setting the variables.

**********************************************************************
** Visual Studio 2017 Developer Command Prompt v15.0
** Copyright (c) 2017 Microsoft Corporation
**********************************************************************

oh oh... You need to run this command from x64 Native Tools Command Prompt for VS 2017.

C:\Users\travis\build\rajat2004\AirSim>REM //---------- copy binaries and include for MavLinkCom in deps ---------- 

C:\Users\travis\build\rajat2004\AirSim>msbuild AirLibWrapper\AirsimWrapper.sln  /target:Clean /target:Build  /property:Configuration=Release /property:Platform=x64 
Microsoft (R) Build Engine version 15.9.21+g9802d43bc3 for .NET Framework
Copyright (C) Microsoft Corporation. All rights reserved.

MSBUILD : error MSB1009: Project file does not exist.
Switch: AirLibWrapper\AirsimWrapper.sln

C:\Users\travis\build\rajat2004\AirSim>if ERRORLEVEL 1 goto :buildfailed 

C:\Users\travis\build\rajat2004\AirSim>echo( 
C:\Users\travis\build\rajat2004\AirSim>echo #### Build failed - see messages above.  1>&2 
C:\Users\travis\build\rajat2004\AirSim>exit /b 1 

The command "if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then docker exec -t bionic /build/setup.sh; docker exec -t bionic /build/build.sh; elif [[ "$TRAVIS_OS_NAME" == "windows" ]]; then export PATH=$MSBUILD_PATH:$PATH; "C:\Program Files (x86)\Microsoft Visual Studio\2017\BuildTools\Common7\Tools\VsDevCmd.bat" -arch=x64; .\\build.cmd; .\\Unity\\build.cmd; elif  [[ "$TRAVIS_OS_NAME" == "osx" ]]; then git submodule update --init --recursive; ./setup.sh; ./build.sh; fi" exited with 1.

Done. Your build exited with 1.
#### Build failed - see messages above. 

[rajat2004]

Closing in favour of #2356