Update ddtrace version for 0.26

[majorgreys]

Upgrading to 0.26.0

This release introduces some important core improvements and continues addressing pain points in our tooling and testing.

Changes

Core

• Add a PeriodicWorker base class for periodic tasks (Add a PeriodicWorker base class for periodic tasks #934)
• Fix runtime workers not flushing to Dogstatsd (Fix runtime workers not flushing to Dogstatsd #939)
• api: simplify _put codepath (api: simplify _put codepath #956)
• make psutil requirement more accurate (make psutil requirement more accurate #949 -- thanks @chrono)
• writer: log a message when a trace is dropped (writer: log a message when a trace is dropped #942)
• span: use system random source to generate span id (span: use system random source to generate span id #940)
• [core] Add config to set hostname tag on trace root span ([core] Add config to set hostname tag on trace root span #938)

Integrations

• Support keyword 'target' parameter when wrapping GRPC channels (Support keyword 'target' parameter when wrapping GRPC channels #946 -- thanks @asnr)
• Record HTTP status code correctly when using abort() with Bottle (Record HTTP status code correctly when using abort() with Bottle #943 -- thanks @equake)
• boto: add support for Python 3.5+ (boto: add support for Python 3.5+ #930)

Documentation

• fix documentation for current_root_span (fix documentation for current_root_span #950 -- thanks @chrono)

Tooling

• Run flake8 with Python 3 (Run flake8 with Python 3 #957)
• tox: fix ignore path for integrations (tox: fix ignore path for integrations #954)
• Remove mention of -dev branch in CircleCI (Remove mention of -dev branch in CircleCI #931)

Testing

• [tests] Add benchmarks ([tests] Add benchmarks #952)
• [tests] increase deviation for sampler by service test ([tests] increase deviation for sampler by service test #948)
• [tests] Fix thread synchronization ([tests] Fix thread synchronization #947)
• [tests] fix brittle deviation test for tracer ([tests] fix brittle deviation test for tracer #945)
• [tests] fix threading synchronization to opentracer test ([tests] fix threading synchronization to opentracer test #944
• [pyramid] Fix dotted name for autopatched config test ([pyramid] Fix dotted name for autopatched config test #932)
• tests: always skip sdist, use develop mode (tests: always skip sdist, use develop mode #928)