workspace: Add ruby interpreter

[jwnimmer-tri]

This will be used first by the sdformat library, but in general is the standard interpreted language used by OSRF / ROS so may have additional uses in the future, such as *.rsdf files.

See #8975 for the PR that puts this to use.

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This change is 

[jwnimmer-tri]

@drake-jenkins-bot linux-xenial-unprovisioned-clang-bazel-experimental please
@drake-jenkins-bot linux-xenial-unprovisioned-gcc-cmake-experimental please

[jwnimmer-tri]

+@jamiesnape for feature review, please.
+@SeanCurtis-TRI for platform review per schedule, please.

[jamiesnape]

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Reviewed 5 of 5 files at r1.
Review status: all discussions resolved, LGTM missing from assignee seancurtis-tri, platform LGTM missing

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tools/workspace/ruby/repository.bzl, line 6 at r1 (raw file):

def ruby_repository(name):
    # Find the ruby binary on $PATH.

BTW This is not entirely true per the documentation of which_repository()

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Comments from Reviewable

[jwnimmer-tri]

Review status: all discussions resolved, LGTM missing from assignee seancurtis-tri, platform LGTM missing

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tools/workspace/ruby/repository.bzl, line 6 at r1 (raw file):

Previously, jamiesnape (Jamie Snape) wrote…

BTW This is not entirely true per the documentation of which_repository()

Done.

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Comments from Reviewable

[jamiesnape]

Reviewed 3 of 3 files at r2.
Review status: all discussions resolved, LGTM missing from assignee seancurtis-tri, platform LGTM missing

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Comments from Reviewable

[jwnimmer-tri]

@drake-jenkins-bot linux-xenial-unprovisioned-clang-bazel-experimental please
@drake-jenkins-bot linux-xenial-unprovisioned-gcc-cmake-experimental please

[amcastro-tri]

Review status: all discussions resolved, LGTM missing from assignee seancurtis-tri, platform LGTM missing

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a discussion (no related file):
Awesome! thanks @jwnimmer-tri!

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Comments from Reviewable

[SeanCurtis-TRI]

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Reviewed 4 of 5 files at r1, 3 of 3 files at r2.
Review status: complete! all discussions resolved, platform LGTM from [seancurtis-tri]

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Comments from Reviewable