tz_convert error when using GMT timezones
#### Code Sample

```python

dates = pd.date_range(start=pd.datetime(2018,1,1), periods=4, freq='h', tz='UTC')

print(dates)
print(dates.tz_convert('Etc/GMT+1'))
print(dates.tz_convert('Europe/Rome'))
```

output:

```
DatetimeIndex(['2018-01-01 00:00:00+00:00', '2018-01-01 01:00:00+00:00',
               '2018-01-01 02:00:00+00:00', '2018-01-01 03:00:00+00:00'],
              dtype='datetime64[ns, UTC]', freq='H')
DatetimeIndex(['2017-12-31 23:00:00-01:00', '2018-01-01 00:00:00-01:00',
               '2018-01-01 01:00:00-01:00', '2018-01-01 02:00:00-01:00'],
              dtype='datetime64[ns, Etc/GMT+1]', freq='H')
DatetimeIndex(['2018-01-01 01:00:00+01:00', '2018-01-01 02:00:00+01:00',
               '2018-01-01 03:00:00+01:00', '2018-01-01 04:00:00+01:00'],
              dtype='datetime64[ns, Europe/Rome]', freq='H')
```

#### Problem description

Converting midnight UTC (`'2018-01-01 00:00:00+00:00'`) to GMT+1 (`Etc/GMT+1`) should return 1 AM (`'2018-01-01 01:00:00+01:00'`), but returns 11 PM (`'2017-12-31 23:00:00-01:00'`), which is one hour behind instead of one hour ahead.

Using `Europe/Rome` returns the correct output, so I tend to think this is specifically related to time zones using `GMT` as reference.

Looking at the outputs, it seems the error might actually come from the `numpy` `datetime64` type, which has `-01:00` as timezone indicator for `GMT+1`.

Maybe I am missing something, since it is very odd this hasn't been spotted before.

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.5.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 78 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.23.0
pytest: 3.0.7
pip: 10.0.1
setuptools: 38.5.1
Cython: 0.25.2
numpy: 1.13.3
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 5.3.0
sphinx: 1.5.6
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.4
feather: None
matplotlib: 2.2.0
openpyxl: 2.4.7
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: 0.9.6
lxml: 3.7.3
bs4: 4.6.0
html5lib: 0.999
sqlalchemy: 1.1.9
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
