transport: default ELASTIC_APM_SERVER_URL

[axw]

If ELASTIC_APM_SERVER_URL is not specified,
default to http://localhost:8200 like the
other agents.

Fixes #121

[codecov-io]

Codecov Report

Merging #122 into master will decrease coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #122      +/-   ##
==========================================
- Coverage   75.57%   75.47%   -0.11%     
==========================================
  Files          61       61              
  Lines        3112     3107       -5     
==========================================
- Hits         2352     2345       -7     
- Misses        577      578       +1     
- Partials      183      184       +1

Impacted Files	Coverage Δ
transport/default.go	84.61% <100%> (-2.89%)	⬇️
transport/http.go	88.18% <100%> (-0.22%)	⬇️
tracer.go	85.66% <0%> (-0.74%)	⬇️

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