Update ARM rootfs urls to nest version inside name

[aneeshusa]

This allows updating each of the rootfs tarballs separately.

Note that the exact format of the URLs is open to some bikeshedding, but I recommend this for the following reasons:

• Switching the version and the name (so that version is inside name) lets you update the tarballs separately
• Keeping the version in the actual file name lets you download different versions without them clobbering each other on disk

cc @larsbergstrom

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This change is

[larsbergstrom]

@bors-servo r+

[bors-servo]

📌 Commit 5856d1a has been approved by larsbergstrom

[bors-servo]

⌛ Testing commit 5856d1a with merge 2c73ca6...

[bors-servo]

☀️ Test successful - travis

[aneeshusa]

@larsbergstrom Wait, did you update the URLs in the S3 bucket to match? I'm not seeing the updated URLs and it failed on Travis too; it only passed because of #274.

[larsbergstrom]

hrm, I comletely misread https://servo-rust.s3.amazonaws.com/ARM/{{ target.download_name }}/{{ target.version }}/{{ target.download_name }}-{{ target.version }}.tgz

Is that what we want? Doesn't target.download_name already have the .tgz on it? Urgh, I was so carefully checking that all of the removed bits were present in the new bits that I missed what the new bits actually said.

/me revokes own r+ privs

[aneeshusa]

Sorry I wasn't more explicit about needing to update the S3 bucket!

You're right about the .tgz extension; I meant to do that and forgot to update it when I moved it over to the map.jinja file. Let me open another PR to fix it.

[larsbergstrom]

@aneeshusa no need to apologize - I need to pay better attention. Thanks! We'll can avoid highstate'ing until the new PR lands and I do update the s3 bucket.

CC @edunham