Why does pandas 0.21.0 require so much downgrading of packages on conda?
This is what conda tells me when I'm trying to upgrade pandas from 0.20.3 to 0.21.0:
```
The following packages will be UPDATED:

    bzip2:      1.0.6-vc14hdec8e7a_1  --> 1.0.6-vc14_1      conda-forge [vc14]
    hdf5:       1.10.1-vc14hb361328_0 --> 1.10.1-vc14_0     conda-forge [vc14]
    jpeg:       9b-vc14h4d7706e_1     --> 9b-vc14_1         conda-forge [vc14]
    libxslt:    1.1.29-vc14hf85b8d4_5 --> 1.1.29-vc14_5     conda-forge [vc14]
    pillow:     4.2.1-py36hdb25ab2_0  --> 4.2.1-py36_0      conda-forge

The following packages will be DOWNGRADED:

    curl:       7.55.1-vc14hdaba4a4_3 --> 7.49.1-vc14_2     conda-forge [vc14]
    freetype:   2.8-vc14h17c9bdf_0    --> 2.7-vc14_1        conda-forge [vc14]
    icu:        58.2-vc14hc45fdbb_0   --> 58.1-vc14_1       conda-forge [vc14]
    libiconv:   1.15-vc14h29686d3_5   --> 1.14-vc14_4       conda-forge [vc14]
    libpng:     1.6.32-vc14hce43e6c_2 --> 1.6.28-vc14_0     conda-forge [vc14]
    libssh2:    1.8.0-vc14hcf584a9_2  --> 1.8.0-vc14_1      conda-forge [vc14]
    libtiff:    4.0.8-vc14h04e2a1e_10 --> 4.0.6-vc14_7      conda-forge [vc14]
    matplotlib: 2.1.0-py36h11b4b9c_0  --> 2.0.2-np113py36_0
    openssl:    1.0.2l-vc14hcac20b0_2 --> 1.0.2l-vc14_0     conda-forge [vc14]
    pycurl:     7.43.0-py36h086bf4c_3 --> 7.43.0-py36_1
    qt:         5.6.2-vc14h6f8c307_12 --> 5.6.2-vc14_1      conda-forge [vc14]
    sqlite:     3.20.1-vc14h7ce8c62_1 --> 3.19.3-vc14_0     conda-forge [vc14]
    tk:         8.6.7-vc14hb68737d_1  --> 8.5.19-vc14_1     conda-forge [vc14]
    yaml:       0.1.7-vc14hb31d195_1  --> 0.1.6-vc14_0      conda-forge [vc14]
    zlib:       1.2.11-vc14h1cdd9ab_1 --> 1.2.11-vc14_0     conda-forge [vc14]
```

Why does it even want to downgrade `matplotlib`?

Why doesn't it upgrade `pillow` to its actual current version `4.3.0`?

I'm don't know if this is even a pandas issue, because this very same pattern now happens when I try to upgrade other packages like `pillow` or `h5py`, but maybe someone can shed some light on this issue, because it's always the same packages that need downgrading, regardless of the package I'm trying to upgrade. And it seems to happen more frequently now, like it seems I can barely upgrade any package without that weird mass downgrade.