Remove trailing slashes from third-party dependency URLs

[mcpwlk]

Base URLs for asyncbigtable and asynccassandra have trailing slashes:

$ git grep '^[A-Z]*_BASE_URL.*/$'
third_party/asyncbigtable/include.mk:ASYNCBIGTABLE_BASE_URL := https://oss.sonatype.org/content/repositories/releases/com/pythian/opentsdb/asyncbigtable/0.3.0/
third_party/asynccassandra/include.mk:ASYNCCASSANDRA_BASE_URL := https://oss.sonatype.org/content/repositories/snapshots/net/opentsdb/asynccassandra/0.0.1-SNAPSHOT/

This causes the build scripts to generate and use URLs with double slashes, like

https://repo1.maven.org/content/repositories/releases/com/pythian/opentsdb/asyncbigtable/0.3.0//asyncbigtable-0.3.0-jar-with-dependencies.jar

This was the cause of a build failure in our system, because the repo1.maven.org repo stopped merging slashes in URLs:

$ curl -Is https://repo1.maven.org/content/repositories/releases/com/pythian/opentsdb/asyncbigtable/0.3.0//asyncbigtable-0.3.0-jar-with-dependencies.jar | head -1
HTTP/1.1 404 Not Found
$ curl -Is https://repo1.maven.org/content/repositories/releases/com/pythian/opentsdb/asyncbigtable/0.3.0/asyncbigtable-0.3.0-jar-with-dependencies.jar | head -1
HTTP/1.1 200 OK

This is a repo admin's fault, however I believe it would be beneficial to remove the trailing slashes for the remaining dependencies, in order to make the build process more independent of third-party repo configurations.

[mcpwlk]

Note that this happened for v2.3.1. In the current master, the URL doesn't redirect to repo1.maven.org, and double slashes don't cause any issues.

[mcpwlk]

Note that there is another hidden issue here, since that specific link under https://oss.sonatype.org/ redirects to https://repo1.maven.org/..., which isn't handled by the build scripts. We work around it by setting CURL="/usr/bin/curl -L --proto-redir =https" as a build variable.