Drop dependency to env_logger

From the looks of it, proc-macro crates' dependencies are treated a bit
differently by Cargo than those of library crates. In particular, they
are not "inherited" by (and, hence, available in) the consuming crate.
As such, the dependency to env_logger that we had specified all along
was effectively unused: while we emitted code that requires the
dependency to be available, it still had to be specified explicitly in
the crate making use of test-env-log.
This change fixes the confusion by converting the dependency to a
dev-only one. We require still require it for testing because our tests
execute the emitted code.

CHANGELOG.md

@@ -1,3 +1,8 @@
+Unreleased
+----------
+- Dropped `env_logger` dependency
+
+
 0.2.1
 -----
 - Relicensed project under terms of `Apache-2.0 OR MIT`

Cargo.toml

@@ -32,11 +32,11 @@ gitlab = { repository = "d-e-s-o/test-env-log", branch = "master" }
 proc-macro = true
 
 [dependencies]
-env_logger = {version = "0.7", default-features = false}
 proc-macro2 = "1.0"
 quote = {version = "1.0"}
 syn = {version = "1.0", features = ["full"]}
 
 [dev-dependencies]
+env_logger = {version = "0.7", default-features = false}
 log = "0.4"
 tokio = {version = "0.2", default-features = false, features = ["rt-core", "macros"]}