fix(tracing): use latest version of semconv

All components need to use a consistent `semconv` version or OTel
will emit errors about "cannot merge resource due to conflicting Schema URL".

Switch to the appropriate semconv version, which requires dropping
usage of `httpconv`. Instead, the upstream HTTP client hooks are
used directly. (The lower-level functions are no longer exported by
OTel.)

Signed-off-by: Milas Bowman <milas.bowman@docker.com>

vendor/github.com/felixge/httpsnoop/.travis.yml

@@ -0,0 +1,6 @@
+language: go
+
+go:
+  - 1.6
+  - 1.7
+  - 1.8