Don't fail if multiple core version are found

[facchinm]

use the latest one based on semver a dependency was added (github.com/blang/semver), hopefully we'll use it somwhere else since it's really useful

In this way we get rid of the annoying Platform {0} (package {1}) is unknown, changing it with a warning. The multiple cores is a pre 1.6.7 Board Manager bug so it should slowly fade away.

@matthijskooijman @cmaglie

[ArduinoBot]

:white_check_mark: Build completed.

:arrow_down: Build URL: http://downloads.arduino.cc/PR/arduino-builder/arduino-builder-84.zip

:information_source: To test this build:

1. Replace arduino-builder binary (you can find it where you installed the IDE) with the provided one

[ArduinoBot]

:white_check_mark: Build completed.

:arrow_down: Build URL: http://downloads.arduino.cc/PR/arduino-builder/arduino-builder-84.zip

:information_source: To test this build:

1. Replace arduino-builder binary (you can find it where you installed the IDE) with the provided one

[ArduinoBot]

:white_check_mark: Build completed.

:arrow_down: Build URL: http://downloads.arduino.cc/PR/arduino-builder/arduino-builder-84.zip

:information_source: To test this build:

1. Replace arduino-builder binary (you can find it where you installed the IDE) with the provided one