Update visual tests

[yuriy-fix]

Visual tests configs were included during the alignment PR #61 . This PR is actually adding tests and fixing errors with the CI build.
Connected to #57

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This change is 

[tomivirkki]

.travis.yml, line 21 at r1 (raw file):

script:
  - if [[ "$POLYMER" = "2" ]]; then
      npm -q i && npm i -q --no-save bower polymer-cli wct-istanbul && bower -q i &&

None of the other components have this. Why make this an exception? Seems like the following is missing https://github.com/vaadin/vaadin-element-skeleton/blob/master/wct.conf.js#L7

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Comments from Reviewable

[yuriy-fix]

Review status: 0 of 12 files reviewed, 1 unresolved discussion (waiting on @YuriyVaadin)

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.travis.yml, line 21 at r1 (raw file):

Previously, tomivirkki (Tomi Virkki) wrote…

None of the other components have this. Why make this an exception? Seems like the following is missing https://github.com/vaadin/vaadin-element-skeleton/blob/master/wct.conf.js#L7

Updated. Thanks!

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Comments from Reviewable

[tomivirkki]

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Reviewed 10 of 12 files at r1, 2 of 2 files at r2.
Review status: complete! all files reviewed, all discussions resolved

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Comments from Reviewable