Add continuous integration testing with travis

[4-20ma]

ModbusMaster version

0.11.0

Arduino IDE version

1.6

Arduino Hardware

all

Platform Details

any

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Feature Request

Narrative:

As a project owner
I want all changes to compile without error
So that end users have a working library

Acceptance Criteria:

Scenario 1: Build is successful
Given a user submits a pull request
When it compiles without error on the automated ci tool
Then it is flagged as passing the build test

Scenario 1: Build fails
Given a user submits a pull request
When the build fails on the automated ci tool
Then it is flagged as failing the build test

[4-20ma]

Refer to WiFiManager for example .travis.yml for Arduino testing.