Consider removal of dependency on sys_info crate

[mikebirdgeneau]

Summary

The sys_info crate is used to determine the current OS for installing the web driver.

• I ran into issues cross-compiling, and noted that since we were not using sys_info for anything other than determining the OS for the webdriver download, we could simply replace this functionality with: std::env::consts::OS, removing this dependency.
• This solves my cross-compilation issue, and can simplify crate dependencies.

Feel free to incorporate if you see fit!

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Tests

   Finished test [unoptimized + debuginfo] target(s) in 0.07s
     Running unittests src/lib.rs (target/debug/deps/webdriver_install-504fbf892f449025)

running 5 tests
test geckodriver::direct_download_url_test ... ok
test chromedriver::direct_download_url_test ... ok
test chromedriver::version_from_output_panic_not_4_parts_test - should panic ... ok
test chromedriver::version_from_output_panic_test - should panic ... ok
test chromedriver::version_from_output_test ... ok

test result: ok. 5 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.01s

     Running unittests src/main.rs (target/debug/deps/webdriver_install-d9ed1320473136de)

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

     Running tests/integration_test.rs (target/debug/deps/integration_test-8874bc6350bfbb78)

running 2 tests
test chromedriver_install_test ... ok
test geckodriver_install_test ... ok

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 3.93s

   Doc-tests webdriver_install

running 4 tests
test src/lib.rs - (line 22) - compile ... ok
test src/lib.rs - (line 7) - compile ... ok
test src/installer.rs - installer::Driver::install_into (line 51) - compile ... ok
test src/installer.rs - installer::Driver::install (line 29) - compile ... ok

test result: ok. 4 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.04s

Dry Run

Dry run (cargo release alpha --dry-run) runs successfully per CONTRIBUTING.md

[phansch]

Thanks, looks good to me! I'll get this merged and released once #59 and #60 are done.