[openstreetmap-website] use ruby 2.1.8 on travis (#1130)

[Tom Hughes]

Merged as .17d166b20f0d914be6e971b3a8677e74f927d4ac

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Reply to this email directly or view it on GitHub:
https://github.com/openstreetmap/openstreetmap-website/pull/1130#issuecomment-171323471
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